Only in a small handful of states (CA, MN, PA, WI, CT):

https://www.aarp.org/work/job-search/info-2021/employers-age...

For the rest of the US, age is only a protected status if you are >40.

Question is how do you prove it? And what can you do about it? A few years back a recruiter was gushing at my resume and said the hiring manager wants to talk to you but wants to know a particular gap in my resume when I left prior employer X. I told the recruiter that I was not happy that X made it very hard for me to take my FMLA leave when my kid was born.

The recruiter hung up shortly and a couple of days later I got the generic rejection letter. I responded by stating that this was discrimination & illegal but I don't have any material evidence and the recruiter simply rejected my claim. So I thought I would post my experience on glassdoor. Couple of days later, I see that my review about the company had been removed from Glassdoor. Glassdoor definitely know which side of one's bread is buttered.

> In the U.S. it's illegal to ask an applicant's age,

Common misconception, but it’s not true except in a few states.

Age discrimination has a very specific legal meaning, and it doesn’t apply at all below a certain age threshold.

Even when age discrimination is in play, simple asking about age (except in those few states) doesn't constitute discrimination. You have to prove that age was a deciding factor in choosing against the hire, which is actually much more difficult than even just proving the question was asked.

In practice, without a ton of leg work to find patterns, how would you prove it?